Running Update Manager produced the error:
Could not download all repository indexes Repository 'https://deb.opera.com/opera-stable stable InRelease' changed its 'Origin' value from 'Opera Software ASA' to 'Opera Software AS'
The fix was simply to run the command:
sudo apt update
This modified the list of repositories and cleared the error.
How to solve the “changed its ‘Origin’ value from ‘Google, Inc.’ to ‘Google LLC’” APT error